GPS, which stands for Global Positioning System, is a geolocation system designed to determine the position of a receiver unit on the Earth anytime, in any weather, anywhere. GPS has 3 parts: the space segment, the user segment, and the control segment. The space segment consists of at least 24 satellites, each in its own orbit above the Earth. The user segment consists of small receivers. The control segment consists of ground stations (five of them, located around the world) that make sure the satellites are working properly.
The GPS satellites each take 12 hours to orbit the Earth. Each satellite is equipped with an accurate clock to let it broadcast signals coupled with a precise time message. A receiver receives the satellite signal, which travels at the speed of light. Even at this speed, the signal takes a measurable amount of time to reach the receiver. The difference between the time the signal is sent and the time it is received, multiplied by the speed of light, enables the receiver to calculate the distance to the satellite. To measure precise latitude, longitude, and altitude, the receiver measures the time it took for the signals from four separate satellites to get to the receiver.
GPS receivers can be hand carried or installed on aircraft, ships, tanks, submarines, cars, and trucks. Many different receiver models are in use. The typical hand-held receiver is about the size of a cellular telephone, and newer models are even smaller.
The GPS signals to a GPS receiver are fairly weak, and significant amounts of processing power are required to acquire and track them. When the receiver is indoors, the signals are considerably attenuated by structure such as walls and ceilings, and the signal strengths may easily drop below the receiver's sensitivity level. As a result, current GPS receivers do not work indoors and often fail or experience severe accuracy degradation in urban environments or under dense foliage.
Various approaches have been developed to increase the sensitivity of GPS receivers so that they may be used indoors. “Assisting data”, such as techniques for weak signal detection, along with efficient correlation and synchronization algorithms, may be delivered to the receiver. One approach, used for cellular handsets that include a GPS receiver as well as the cellular signal receiver, is to pass assisting data from a cell site to the handset via the cellular system. The data is then passed to the GPS receiver. A second approach is the use of differential GPS (psuedolite system), which broadcasts correction data to GPS receivers. A third approach uses a wide area augmentation system (WAAS), specifically for aiding aircraft during landing. Each of these systems requires receiver hardware in addition to that used for acquiring and processing navigation data.